I emailed Stihl about this.
There reply,
Hello Mr. XXXX,
We are in receipt of and thank you for your e-mail.
Stabilizer was deleted from the refining process of STIHL HP Ultra engine oil because stabilizers only have a marginal effect regarding shelf life extension of mixed fuel. Before modern fuels were augmented with the additives currently used, stabilizers were a much more effective product regarding extending the shelf life of gasoline. Nowadays, stabilizers only extend the shelf life for a matter of days rather than weeks or months.
Thank you again for your e-mail.
Best regards,
Wayne Lemmond
Technical Service Representative
STIHL Inc.
:)